1. Field of the Invention
The present invention relates to a cold-rolled steel sheet and a method for manufacturing it, and relates specifically to a cold-rolled steel sheet with high-strength of 980 MPa or above tensile strength and a method for manufacturing it.
2. Description of the Related Art
The automobile industry is facing an urgent need of responding to global environmental issues such as CO2 discharge regulation. On the other hand, from the viewpoint of securing safety of passengers, the safety standards for collision of automobiles have been made stricter, and structural designing sufficiently securing safety in the occupied space has been progressing. In order to achieve these requirements simultaneously, it is effective to use a high-strength steel sheet (high-ten) with 980 MPa or above tensile strength as a structural member for an automobile and to make the thickness further thinner to reduce the vehicle bodyweight. However, in general, when the strength of a steel sheet is increased, the workability deteriorates, and therefore improvement of the workability is an unavoidable problem in order to apply a high-ten to members for automobiles.
As a steel sheet provided with both the strength and workability, TRIP (Transformation Induced Plasticity) steel sheets are known. As one of the TRIP steel sheets, a TBF steel sheet with a base metal phase of bainitic ferrite including residual austenite (may be hereinafter referred to as “residual γ” is known (Japanese Unexamined Patent Application Publication Nos. 2005-240178, 2006-274417, 2007-321236, and 2007-321237). With respect to the TBF steel sheet, high strength can be obtained by the presence of hard bainitic ferrite, and excellent elongation (EL) and stretch flange formability (λ) can be obtained by the presence of fine residual γ present on the boundary of bainitic ferrite, and therefore both of the strength and excellent workability can be achieved.
However, CO2 discharge regulation has become more severe in recent years, and further reduction of the vehicle body weight has been required. Accordingly, even for a difficult-to-form member for which a low strength steel sheet with the excellent workability was used conventionally, application of a high-ten with 980 MPa or above tensile strength has been studied. More specifically, positive use of the high-ten has been considered not only for frame members of the vehicle body but also for seat members and the like.
As described above, the steel sheet used as a member for automobiles is required to be provided with both the high strength and workability. Particularly recently, further improvement of the overall workability including the local deformation performance such as the stretch flange formability (bore expandability) and bending properties in addition to the elongation has been highly required for a 980 MPa or above high-strength steel sheet whose applied amount has been increasing.